Add CPU operation callback when the CPU is stopped

The KVM clock needs this callback to clear its MSR when the CPU is
stopped so the host does not continue to write into the shared area.
Include the signature in the hypervisor info structure
Various cleanup
Add code to calculate the TSC frequency from the KVM/PV clock

Not currently used. Could later be used similarly to the VMware code
to avoid attempting to calibrate the TSC.
Add basic bhyve guest detection